Atmospheric Pressure Ionization (API) Sources including Electrospray (ES), Atmospheric Pressure Chemical Ionization (APCI) and Inductively Coupled Plasma (ICP) ion sources interfaced to mass analyzers are typically operated with a single sample introduction probe. In mass spectrometric applications where internal standards are required, additional components can be added to the primary sample solution where the resulting mixture is delivered through one probe into the API source. The mixture of compounds in a single solution introduced through the same probe are ionized and mass analyzed. A known sample when mixed with an unknown sample can serve as an internal mass scale or quantitation calibration standard for the unknown components peaks appearing in the mass spectrum acquired in this manner. However, mixing a known compound calibration solution with an unknown sample solution can have undesired analytical consequences. The known and unknown solution components may effect one another in an unpredictable manner during the solution transport or ionization process. One component may react with another in solution or one or more components may suppress the ionization efficiency of other components during the ionization process. A solution with a known component mixture may be difficult to eliminate as a source of chemical contamination in a probe which is running a series of unknown samples at the trace component level. If it is desirable to deliver a known solution as a mixture through the sample introduction probe on an intermittent basis, the occasional sample introduction will be subject to the constraints of solution flow rates through the probe, efficiency of mixing solutions, dead volume losses and flushing of the probe to eliminate the known solution prior to the next analysis. The invention avoids performance and sample introduction problems encountered when mixing liquid samples prior to ionization in an API source, by conducting simultaneous mass analysis of two different solutions without the need to mix solutions in the same probe prior to analysis. One aspect of the invention is the configuration and simultaneous operation of multiple probes or multiple sprayers or nebulizers within a probe assembly through which different sample solutions can be introduced simultaneously into an API source during operation.
In one embodiment of the invention, multiple sample introduction means have been configured in Electrospray Atmospheric Pressure Ion sources which are interfaced to mass analyzers. At least two sample introduction Electrospray probes are operated simultaneously in an Electrospray ion source. At least one ES probe is supplied a sample which is different from the sample solution supplied to additional ES probes operating within the same ES source chamber. In this manner a calibration solution can be introduced through one ES probe while an unknown sample is introduced through another ES probe or second channel within the same ES probe assembly. Ions produced from both solutions via the simultaneous spraying of both ES probes blend or mix in the atmospheric pressure ES chamber background gas prior to entering the orifice into vacuum. The mixture of ions resulting from the solutions delivered from at least two ES probes is simultaneously mass to charge (m/z) analyzed resulting in a mass spectrum containing an internal standard for calibrating or tuning the mass analyzer. The internal calibration standard contained within the acquired mass spectrum is achieved without mixing known and unknown samples in solution. Simultaneous introduction of different samples through multiple ES probes also enables the study of mixed ion and molecule reactions at atmospheric pressure in the ES source chamber prior to introduction into vacuum. Each ES sample introduction probe assembly can be configured with nebulization gas and liquid layered flow. An internal calibration solution can be included in the layered flow or the primary flow of any given ES probe configured in the ES source chamber. The individual sample solution flows or nebulization gas flows to any combination of ES probes can be switched on or off during an analytical run without the need to reposition probes. In another aspect of the invention, an Atmospheric Pressure Chemical Ionization (APCI) source assembly can be configured with multiple inlet channels or probes. These multiple APCI inlet probes can include pneumatic nebulization and the solution and gas flow supplied to each inlet probe can be individually or simultaneously turned on or off. In both the ES and APCI sources, multiple probe sample solution ionization can be controlled without the need to reposition probes by switching voltages, controlling the nebulization gas flows or controlling the sample solution flows. Configurations of multiple sample introduction inlet probes can also be extended to a system that has a combination of both Electrospray and APCI ion production means in the same API chamber. Each ES or APCI sample inlet probe can include pneumatic or ultrasonic nebulization.
Configurations of Electrospray ion sources which include more than one sample introduction needle or nebulizer have been described in the literature. Kostianinen and Bruins, Proceedings of the 41st ASMS Conference on Mass Spectrometry, 744a, 1993, described the configuration and use of an assembly of multiple Electrospray inlet tips with and without pneumatic nebulization mounted in an Electrospray ion source. Each ES tip was supplied the same sample solution delivered from a single pump with a single solution source. The sample solution, delivered from a liquid chromatography pump, flowed into an assembly or array of one, two or four ES or pneumatic nebulization assisted ES sprayer tips in an attempt to improve ion signal intensity at higher liquid flow rates. In the arrangement reported, the solution flow to individual sprayer tips could not be turned on and off independently and different solutions could not be introduced selectively to individual sprayer tips in the assembly of multiple ES sprayer tips.
Rachel R. Ogorzalek Loo, Harold R. Udseth, and Richard D. Smith, Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics, 266-267, 1991 and J. Phys. Chem., 6412-6415, 1991 and Richard D. Smith, Joseph A. Loo, Rachel R. Ogorzalek Loo, Mark Busman, and Harold R. Udseth, Mass Spectrometry Reviews, 10, 359-451,1991 describe the configuration of an Electrospray ion source interfaced to a quadruple mass analyzer apparatus which included dual Electrospray ion sources delivering ions to two separate entrance apertures of a Y shaped capillary. Positive ions created in one Electrospray source were introduced into one inlet branch of the Y shaped capillary and negative ions created from the second Electrospray ion source were introduced into the second inlet branch of the Y shaped capillary. The positive and negative ions swept into the two entrance orifices of the capillary tube began mixing where the two inlet branches of the capillary tube met well downstream of the capillary entrances located in the two ES atmospheric pressure source chambers. Dual Electrospray ionization sources or a separate ES source and a gas phase corona discharge source individually delivered ions into two entrance orifices of a Y shaped capillary. For all experiments reported, the first ES source produced ions of opposite polarity to the second ES or gas phase corona discharge source. The opposite polarity ions produced in separate ion sources were not mixed in the atmospheric pressure ion source but entered a split capillary tube at two separate entrance orifices and mixed in partial vacuum downstream in the capillary tube.
Bordoli, Woolfit and Bateman, Proceedings of the 43th ASMS Conference on Mass Spectrometry and Allied Topics, 98, 1995 described an Electrospray ion source which included a calibration ES probe configured with a second microtip (50 nl/min flow rate) sample probe interfaced to a magnetic sector mass analyzer. The sample probe included a microtip attached directly to a syringe needle. The syringe was mounted on an X-Y-Z positioning stage to optimize the position of the microtip sprayer. The calibration ES probe was configured such that it could be moved into a position when a calibration solution was sprayed at 500 nl/min while no sample flowed through the primary ES sample probe. After acquisition of a calibration mass spectrum, the calibration ES probe was retracted and the calibration solution flow turned off. The sample flow through the microtip sample ES probe was then turned on and a separate mass spectrum was acquired from the Electrosprayed ions produced. In this manner, an external calibration mass spectrum was acquired prior to acquisition of a mass spectrum of the primary sample. The calibration mass spectrum and the sample mass spectrum were then added together in the data system prior to calculating the mass assignment of the sample related peaks. For the ES source configuration reported, the two ES probes were not operated simultaneously and no gas phase mixture of calibration and sample ions was created at atmospheric pressure and no mass spectrum was acquired from a mixture of calibration and sample ions. No single mass spectrum was acquired which included sample related peaks and calibration compound related peaks with the apparatus described. Neither ES probe described was configured to operate with pneumatic nebulization assisted Electrospray. The ES calibration probe position required adjustment prior to acquiring a calibration spectrum to enable effective spaying near the orifice into vacuum. After acquisition of a calibration mass spectrum, the ES calibration probe was retracted to avoid interference prior to the mass spectrum acquisition from the sample solution delivered through the primary ES probe.
In one embodiment of the invention described, multiple samples are introduced into an API source simultaneously where ions are produced from all samples and mixed in the atmospheric pressure ion source chamber. A portion of the gas phase ion mixture is then swept into vacuum through an orifice or capillary where the ions are mass analyzed. In this manner a solution containing calibration compounds can be ionized simultaneously with a sample solution resulting in an acquired mass spectrum containing an internal standard without mixing calibration components and sample components in solution. Higher mass accuracy's can be achieved with an internal standard when m/z assignments are calculated for sample ion related peaks in an acquired mass spectrum. In addition to independently introducing calibration compounds in an API source, multiple sample inlet probes can be used to introduce multiple samples individually or simultaneously into an API source. Mounting multiple probes in an API chamber such as ES and APCI probes, allows multiple ionization techniques to be run individually or simultaneously in a single API source assembly. Multiple Electrospray probes can be configured to collectively provide optimal performance over a wide range of sample flow rates and solution chemistries. ES probe positions can be configured to fall directly on the vacuum orifice centerline to a position angled to well over 100 degrees off the centerline. Different liquid flow rates can be delivered to separate ES or APCI probes within the same API source. ES and/or APCI probes mounted at different positions in the ES source chamber, can operate simultaneously, in pairs or in groups at different flow rates and introducing different sample solutions. The multiple ES probes may be operated with or without nebulization assist.